The human and mammal bodies use electrical signals to achieve sensory input, muscle movements, thoughts, and memory. Over time, these signals are also responsible for neural plasticity, which includes general wiring, rewiring, and de-wiring of the brain. The electrical signals are represented in the mind and body as potentials (voltages) created by ions, not electrons. However, these ion-transported signals can be initiated, negated, or altered by electric fields that originate from outside the body. By Faraday's law of electromagnetics, these electric fields can be generated from changing magnetic fields, hence, the name “magnetic stimulation”. Because these signals are initiated from outside the body, magnetic stimulation can be a non-invasive means for altering or improving of almost all bodily and mental functions.
The signals inside the body are “action potentials” that are pulse-frequency modulated, meaning that the pulse rate is related to the intensity of the sensed input, muscular energy, or neuronal message. The shapes of individual pulses are largely the same throughout, having a pulse width of about 1 millisecond and some undershoot after the main pulse. The pulse height is approximately 70 millivolts for sensory signals and somewhat larger for muscle activation. Pulses for the heart, digestive system, and may other organs have other unique characteristics. For the most part, the signals all look the similar when viewed on an oscilloscope: a “pulse train” wherein the pulse repetition frequency is indicative of the magnitude of the transmitted signal. The absence of a pulse train can also cause a reaction, explaining why amputees still feel parts of the body that no longer exist.
The meaning of the individual signal to the body's nervous system is dependent on where pulse train appears. The brain consists of regions that handle various neural functions and provide input for thoughtful and sensory processing. The peripheral nervous system contains axons that serve as communication channels and repeaters between the sensory nerve endings and the spinal cord and ultimately the brain. The neuromuscular system also consists of axons that communicate in the opposite direction allowing the brain to cause various muscle motions. Axons are grouped together into multi-channel peripheral nerves as they approach the spinal cord or the brain. Some axons are myelinated to increase the propagation rate of the pulse trains to and from the extremities of the body.
Neuromodulation devices strive to create, negate, or alter these naturally-occurring pulse trains in a targeted location to achieve a beneficial result. This may include blocking or stimulation of neural activity. Ultimately, an electric field is required at the location that causes ions to appropriately to trigger an action potential that then can propagate unassisted through the nervous system to its destination. This electric field may be induced rather than generated directly. For example, traditional magnetic stimulation first creates a time-varying magnetic field from a coil of wire, which in turn generates an electric field per Faraday's law. When this electric field is induced on a portion of the neurosensory system, or the neuromuscular system, or brain's neural network, it can alter that system by depolarizing or hyper-polarizing the pulse trains that naturally exist or by inserting a pulse train that does not exist. In the nervous system and the brain, these pulse trains run continuously; only the frequency changes to convey the intensity information.
The prior-art neural stimulation devices fall into three categories: (1) implanted wire stimulation wherein electrodes implanted at a targeted location and connected by wires to a driver circuit possibly also implanted in another part of the body, (2) magnetic stimulation wherein changing magnetic fields produced by a coil outside the body generate electric fields inside the body that alters the natural nerve or neuronal signals, and (3) skin-electrode stimulation wherein electrodes are placed on the skin and cause current to flow into the body from one electrode to the other. Deep Brain Stimulation (DBS) is an example of implanted wire stimulation. Transcranial Magnetic Stimulation (TMS) is an example of magnetic stimulation. Transcutaneous Electrical Neural Stimulation (TENS) and Electro Convulsive Therapy (ECT) are examples of skin-electrode stimulation.
Implanted wire stimulation is highly targeted, but also highly invasive and unstable due to electrode movement from wire-tugging during bodily motions. Infection is also a disadvantage especially if the driver circuit is not implanted. The mechanism of action is increasing or decreasing the frequency of natural action potentials and therefore well understood. Examples of implanted wire stimulators include the Vagus nerve stimulator offered by Cyberonics and covered in U.S. Pat. No. 8,571,654B2 that has helical electrodes, and US2016/0175600A1 where the implant includes a battery charged wirelessly by external coils transmitting the recharge energy magnetically. Some implanted wire stimulators have implanted micro-coils that induce electric fields in the body instead of providing voltages on electrodes, such as US2015/0080637.
Magnetic stimulation is non-invasive, but unpredictable and low in efficacy because the stimulation is not targeted and the mechanism of action is not understood. Regarding medical treatment, magnetic stimulation has achieved regulatory approval for treating major depression, neuropathic pain, and headaches. According to clinicaltrials.gov, 1165 clinical studies have been or are being performed with “magnetic stimulation” by 427 unique sponsors to understand its effect on 450 different conditions. Magnetic stimulation may include a single external coil, multiple external coils for better targeting such as US2012/0302821A1 and also wearable coils such as U.S. Pat. No. 9,072,891B1 and US2010/0160712A1.
Skin-electrode stimulation is non-invasive, but untargeted and uncontrollable because the electrical current follows multiple paths with varying intensity. The mechanism of action of skin-electrode stimulation is not understood except for ECT where an electrical Jolt is large enough to intentionally produce a full seizure in the brain. ECT and TENS are approved for very few indications and efficacy is low.